1. Field of the Invention
The present invention relates to a wire electric discharge machine and a wire electric discharge machining method, and more specifically, to a wire electric discharge machine and a wire electric discharge machining method capable of appropriately coping with a machining error attributable to set posture inclination of a workpiece to be machined.
2. Description of the Related Art
In general, in a wire electric discharge machine, a workpiece is set and fixed on a workpiece table that is movable within an XY-plane, and voltage is applied between a wire electrode and the workpiece to perform electric discharge machining. The wire electrode is held by upper and lower wire guides. Normally, the workpiece as an object of wire electric discharge machining has a flat lower surface to be in contact with a mounting surface of the workpiece table and a workpiece surface (whole or part of an upper surface) parallel to the lower surface. If the workpiece has the shape of a rectangular parallelepiped, for example, the whole of its one surface forms the lower surface, and its entire upper surface parallel to the lower surface forms the workpiece surface.
If the workpiece is set and fixed on the workpiece table so that its lower surface is intimately in contact with the mounting surface, both the lower surface of the workpiece and the workpiece surface extend parallel to the XY-plane. If the upper and lower guides are kept in the same positions within the XY-plane in machining, therefore, a machined surface is perpendicular to the XY-plane. The so-called vertical machining is performed in this state. If the upper and lower guides are shifted relatively on the XY-plane, moreover, taper machining can be realized such that the machined surface is inclined at a given angle to the XY-plane.
If the workpiece is actually set on the workpiece table, however, its lower surface sometimes may be unevenly lifted above the mounting surface of the table so that the set posture of the workpiece is inclined with respect to the XY-plane, owing to sludge or the like with which the mounting surface is soiled or fine dust adhering to the lower surface of workpiece. During use of the machine, moreover, the workpiece table itself may possibly be inclined with respect to the XY-plane, caused by collision between the workpiece and the guides, secular change, etc.
The following problems (a) and (b) are aroused if the wire electric discharge machining is executed in this state.
(a) Angle error of machined surface: If vertical machining is expected to be performed with the upper and lower guides kept in the same positions within the XY-plane under the presence of the aforesaid posture error, for example, a vertical machined surface cannot be obtained, and a required taper angle cannot be obtained even by taper machining, in some cases. This phenomenon is a machined surface angle error.
(b) Distortion error of machined shape: Inclination of the workpiece causes a fine shift of a machined point that is ordered by a machining program, and a distortion error occurs as the shift changes along a machined line. For example, an irregularly circular machined line may be obtained even if circular machining is programmed. This shape error may possibly be ignored if the inclination of the workpiece is very small. It is not negligible, however, if the workpiece inclination is large or if high machining accuracy is needed.
Conventionally, in order to avoid these problems, an operator must pay close attention to set the workpiece exactly horizontally with respect to the XY-plane. Further, much labor is required to remove soil on the workpiece table or the mounting surface or to keep the workpiece table itself horizontal or correct its inclination. There is no disclosure of any simple conventional technique that can release the operator from the burden of such labor.